Abdominal aortic aneurysms (AAA) represent one of the most common types of aneurysms and result in about 15,000 deaths annually in the United States. An aneurysm is produced when a thinning or weak spot in a vessel wall dilates eventually posing a health risk from its potential to rupture, clot, or dissect. An aneurysm frequently occurs in arteries, but may also form in veins. The etiology of aneurysm formation is not entirely understood, but is thought to be related to congenital thinning of the artery, atherosclerotic vessel degeneration, vessel trauma, infection, smoking, high blood pressure, and other causes leading to vessel degeneration. Left untreated, AAA may lead to gradual vessel expansion, thrombus formation leading to stroke or other vessel blockage, vessel rupture, shock, and eventual death.
AAA are generally localized on long abdominal aortic sections below the renal arteries and oftentimes extend into one or both of the iliac arteries. The aneurysm may begin with a small vessel distension that progressively enlarges at a variable and unpredictable rate. An AAA may enlarge at an average rate of about 0.3–0.5 cm per year. The AAA may continue to enlarge in a silent fashion until a catastrophic event, such as a rupture, occurs. The best predictor of rupture risk is size, wherein rupture is relatively uncommon in AAA less than 5 cm. Once reaching about 8 cm, however, there is about a 75 percent chance of rupture within a year. Besides rupture, another risk of AAA is thrombus dissection. As the vessel enlarges, a thrombus may develop in the aneurysm due to perturbations in blood flow dynamics. Pieces of the clot may eventually loosen and carry away, eventually forming blockages in the legs, lungs, or brain.
AAA are most commonly treated in open surgical procedures, where the diseased vessel segment is bypassed and repaired with an artificial vascular graft. While considered to be an effective surgical technique, particularly considering the alternative of the usually fatal ruptured AAA, conventional vascular graft surgery suffers from a number of disadvantages. The surgical procedure is complex and requires experienced surgeons and well equipped surgical facilities. Even with the best surgeons and equipment, patients suffering from such aneurysms are often elderly and weakened from cardiovascular and other diseases. This factor reduces the number of patients eligible for surgery. Even for eligible patients prior to rupture, conventional aneurysm repair has a relatively high mortality rate, usually from 2 to 10%. Morbidity related to the conventional surgery includes myocardial infarction, renal failure, impotence, paralysis, and other conditions. Even with successful surgery, recovery takes several weeks and often requires a lengthy hospital stay.
To overcome some of the drawbacks associated with open surgery, a variety of endovascular prosthesis placement techniques have been proposed. Without the need for open abdominal surgery, patient complications and recovery time may be significantly reduced. One endovascular AAA repair technique involves a tubular prosthesis deployed by remote insertion through a femoral artery. The prosthesis may include a synthetic graft sheath body supported by an expandable structure such as a stent. The stent may be self-expanding or balloon-expanding and typically includes means for anchoring the prosthesis to the vessel wall. The stent-graft acts as a shunt to carry blood flow from a healthy portion of the aorta, through the aneurysm, and into one or both of the iliac artery branches. The prosthesis excludes any thrombus present in the aneurysm while providing mechanical reinforcement of the weakened vessel reducing the risk of dissection and rupture, respectively.
A number of endovascular AAA stent-graft prosthesis designs are known. For aneurysms proximal to the iliac arteries, many of the designs utilize bifurcated structures. Bifurcated stent-graft prostheses generally have a trunk portion with a relatively large lumen deployed in the aorta, and first and second branch portions with smaller branch lumens deployed within each of the iliac arteries. The deployed trunk and branch portions preferably seal to each other and to the healthy vascular walls beyond the aneurysm to isolate the aneurysm from the bloodstream. Advantageously, the aortic blood flow enters the trunk prosthetic lumen, is separated into the two branch prosthetic lumens, and then flows into each of the iliac arteries in a path that approximates that of a normal, healthy vascular system.
In certain situations, it is desirable to extend the length of the deployed prosthesis trunk portion and provide a seal further up the aorta, into healthy vascular tissue. Failing to do so may result in leakage into the aneurysm. Trunk extension may be warranted after prosthesis migration resulting from morphological changes in the aneurysm, or during prosthesis deployment in a tortuous vessel. The trunk extension may be performed after the AAA repair procedure, as with prosthesis migration, or during the initial procedure, as with a tortuous vessel. Extension may be accomplished by deploying a tubular shaped extension cuff partially within the trunk body. The deployed extension cuff may extend the effective length of the prosthesis and provide a seal to healthy vascular tissue. Multiple extension cuffs may be used in series to further extend the prosthesis length or to negotiate particularly tortuous vessel paths.
An extension cuff is generally a stent-graft device that allows adjustment of the length of the implanted bifurcated prosthesis. The extension cuff may expand to a diameter slightly larger than the prosthesis trunk portion thereby providing sealed attachment between the trunk and cuff. Frictional forces between the overlapping surfaces usually prevent the cuff from detaching from the primary stent-graft module. At its other end, the extension cuff may seal against healthy vascular tissue minimizing blood flow into the aneurysm. Such deployment of the second branch can be very straightforward and in situ deployment of a bifurcated prostheses and extension cuff appears to hold significant promise for many AAA patients.
One shortcoming associated with the extended prosthetic stent-graft relates to extension cuff separation. The AAA may vary widely in location, size, and the distended shape of the aneurysm itself. Particularly after treatment, the aneurysm and associated vessels may drastically change morphology thereby exerting stress forces on the deployed prosthesis. A significant amount of stress may be exerted on the joint between the trunk portion and the extension cuff. Multiple extension cuffs used in series may be particularly vulnerable to such stresses due to the presence of multiple joints. With sufficient change in aneurysm morphology and subsequent stress placed on the joint(s), the extension cuff may separate from the prosthesis. The patient may have to undergo another treatment given the problem is detected early. Undetected module separation may lead to continued leakage, aneurysm regrowth, and even the more serious problems associated with AAA. Accordingly, it would be advantageous to prevent separation of the extension cuff from the endoluminal prosthesis.
Therefore, it would be desirable to provide a bifurcated endoluminal prosthetic assembly and a method of extending an endoluminal prosthetic assembly that overcomes the aforementioned and other disadvantages.